Concrete Slab Calculator
Calculate cubic yards and premix bags (60lb/80lb) for slabs, footers, sidewalks, and columns. Includes waste factor for accurate ordering.
Enter Your Measurements
Recommended: 10% for most projects
Understanding Slab Dimensions
Length and Width are measured in feet. Depth is measured in inches.
Your Estimate
Total Required
0.00
Cubic Yards
Bag Estimates
Material Breakdown
The Math Behind the Calculator
Concrete Volume Formula
Cubic Yards = (Length x Width x Depth) / 27
Where Length and Width are in feet, and Depth is converted from inches to feet (divide by 12).
The formula works because there are 27 cubic feet in one cubic yard. When ordering ready-mix concrete, suppliers sell by the cubic yard. If you're using bagged concrete from a home improvement store, you'll need to convert using these estimates:
- 80lb bags: Approximately 0.6 cubic feet per bag, or about 45 bags per cubic yard
- 60lb bags: Approximately 0.45 cubic feet per bag, or about 60 bags per cubic yard
Example Calculation
For a 10ft x 10ft slab that is 4 inches deep:
- Convert depth to feet: 4 inches / 12 = 0.333 feet
- Calculate volume: 10 x 10 x 0.333 = 33.3 cubic feet
- Convert to cubic yards: 33.3 / 27 = 1.23 cubic yards
- Add 10% waste factor: 1.23 x 1.10 = 1.36 cubic yards
Concrete Buying Guide: 3000 PSI vs 4000 PSI
3000 PSI Concrete
Standard residential-grade mix suitable for most home projects.
- Best for: Sidewalks, patios, driveways
- Cures to: 3,000 pounds per square inch
- Cost: Most affordable option
- Set time: 24-48 hours to walk on
4000 PSI Concrete
High-strength mix for heavy loads and commercial applications.
- Best for: Garage floors, footings, heavy equipment pads
- Cures to: 4,000 pounds per square inch
- Cost: 10-15% more than 3000 PSI
- Set time: 24-48 hours to walk on
Pro Tip: When to Upgrade
If your slab will support vehicle traffic, consider 4000 PSI. For a shed pad or patio that only sees foot traffic, 3000 PSI is perfectly adequate and saves money.
Pro Tips for Your Concrete Project
Why 10% Waste Factor is Standard
Concrete waste comes from several sources: uneven subgrade, form blowouts, spillage, and the concrete left in the truck or mixer. For complex shapes or sloped ground, consider increasing to 15%.
Rebar Spacing Guidelines
For a 4-inch slab, use #3 rebar (3/8" diameter) spaced 18 inches on center in a grid pattern. Position the rebar in the lower third of the slab using rebar chairs. For driveways or heavy loads, reduce spacing to 12 inches.
Fiber Mesh Alternative
For smaller slabs, synthetic fiber mesh mixed into the concrete can replace rebar for crack control. It won't provide the same structural strength but works well for patios and walkways.
Temperature Matters
Concrete should be poured when temperatures are between 50-85°F (10-29°C). Below 50°F, curing slows dramatically. Above 85°F, rapid water evaporation causes surface cracking.
How to Prepare a Subbase for a Concrete Shed Pad
A proper subbase is crucial for preventing cracks and settling. Follow these steps for a solid foundation:
- Excavate the area - Dig down 6-8 inches below your finished slab height. Make the excavation 6 inches wider than your slab on all sides.
- Compact the soil - Use a plate compactor to compress the native soil. This prevents settling later.
- Add gravel base - Spread 4 inches of crushed gravel (3/4" minus or "road base"). Compact in 2-inch lifts.
- Install forms - Use 2x4 or 2x6 lumber for form boards. Stake every 3 feet and ensure forms are level (or sloped 1/8" per foot for drainage).
- Add vapor barrier - Lay 6-mil polyethylene sheeting over the gravel to prevent moisture from wicking up into the slab.
- Install reinforcement - Place rebar grid or welded wire mesh on rebar chairs to position it in the lower third of the slab.
Common Mistake to Avoid
Never pour concrete directly on unprepared soil or grass. The organic material will decompose, causing voids and eventual cracking. Always remove topsoil and vegetation first.
Safety First
Wear Rubber Boots
Wet concrete is caustic and can cause chemical burns. Never let it contact skin for prolonged periods.
Wear Safety Glasses
Concrete splashes can cause serious eye injuries. Always wear eye protection when mixing or pouring.
Use Waterproof Gloves
Protect your hands from the alkaline properties of cement which can irritate and dry out skin.
Work in Ventilated Areas
When mixing dry concrete, dust can irritate lungs. Wear a dust mask and work outdoors when possible.
Frequently Asked Questions
How thick should a concrete slab be?
For patios and sidewalks, 4 inches is standard. Driveways should be 5-6 inches. Garage floors typically require 4-6 inches depending on vehicle weight.
How long does concrete take to cure?
Concrete reaches about 70% strength in 7 days and full strength in 28 days. You can walk on it after 24-48 hours, but avoid heavy loads for at least a week.
Should I order ready-mix or use bags?
For projects under 1 cubic yard, bags are practical. For 1-3 yards, it's a close call - bags are more work but don't require timing a delivery. Over 3 yards, always order ready-mix.
Do I need a permit to pour a concrete slab?
Requirements vary by location. Generally, small patios under 200 sq ft don't need permits, but detached structures (sheds) or driveways often do. Check with your local building department.